For devices which are based on white light-emitting diodes (LEDs), particularly for backlighting, there are only a few solid phosphors which meet the requirements of an LED phosphor, in particular with emission in the dark red region of the electromagnetic spectrum. Hitherto, use has mainly been made of two orange to red-emitting phosphors of the formula (Sr, Ba)2Si5N8:Eu2+ and (Sr,Ca)AlSiN3:Eu2+. However, these have significant disadvantages with regard to emission, coverage of the color space, half-value width (FWHM=Full Width Half Maximum) and spectral filtering. In the case of the phosphor (Sr,Ba)2Si5N8:Eu, the emission wavelength may be shifted from the orange into the red region of the spectrum by replacing barium with strontium. However, this replacement results in a reduction in the long-term stability of the phosphor. (Sr,Ba)2Si5N8:Eu2+ phosphors additionally exhibit large half-value widths and do not exhibit any emission in the dark red region of the spectrum, i.e., no emission at a dominant wavelength of over 620 nm. Although the phosphor (Sr,Ca)AlSiN3:Eu2+ already exhibits emission in the dark red region of the spectrum, it exhibits very broad emission, which extends into the non-visible region of the electromagnetic spectrum, whereby the luminescence efficiency of this phosphor is reduced. There is therefore considerable demand for a phosphor which exhibits emission in the dark red region of the electromagnetic spectrum and a small half-value width and thereby little emission outside the visible region of the electromagnetic spectrum.
WO 2013/175336 A1 and Nature Materials 2014, P. Pust et al., “Narrow-band red emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material” disclose a phosphor of the formula SrLiAl3N4:Eu2+ which already has an emission in the dark red region of the electromagnetic spectrum and a small half-value width, said phosphor additionally exhibiting little emission outside the visible region of the electromagnetic spectrum. However, the phosphor has a lower quantum efficiency, i.e., a poor ratio between the number of emitted and absorbed photons, compared with (Sr,Ba)2Si5N8:Eu2+ and (Sr,Ca)AlSiN3:Eu2+.